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Laboratory Investigation of the Dynamics of Shear Flows in a Plasma Boundary Layer by Ami Marie DuBois A dissertation submitted to the Graduate Faculty of Auburn University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy Auburn, Alabama December 14, 2013 Copyright 2013 by Ami Marie DuBois Approved by Edward Thomas, Jr., Chair, Professor of Physics William Amatucci, Staff Scientist at Naval Research Laboratory Uwe Konopka, Professor of Physics David Maurer, Professor of Physics Abstract The laboratory experiments presented in this dissertation investigate a regime of instabilities that occur when a highly localized, radial electric field oriented perpendicular to a uniform background magnetic field gives rise to an azimuthal velocity shear profile at the boundary between two interpenetrating plasmas. This investigation is motivated by theoretical predictions which state that plasmas are unstable to transverse and parallel inhomogeneous sheared flows over a very broad frequency range. Shear driven instabilities are commonly observed in the near-Earth space environment when boundary layers, such as the magnetopause and the plasma sheet boundary layer, are compressed by intense solar storms. When the shear scale length is much less than the ion gyro- radius, but greater than the electron gyro-radius, the electrons are magnetized in the shear layer, but the ions are effectively un-magnetized. The resulting shear driven instability, the electron-ion hybrid instability, is investigated in a new interpenetrating plasma configuration in the Auburn Linear EXperiment for Instability Studied (ALEXIS) in the absence of a magnetic field aligned current. In order to truly understand the dynamics at magnetospheric boundary layers, the EIH instability is studied in the presence of a density gradient located at the boundary layer between two plasmas. Theoretical models are used to show that the EIH instability in a uniform density plasma cannot be supported by the parameters that are accessible to ALEXIS. ii As plasma boundary layers begin to relax from a compressed state, the ratio of the ion gyro-radius to the shear scale length decreases, and observations of broadband electrostatic noise, which extend from well below the ion cyclotron frequency to the electron plasma frequency, have been reported. By decreasing the magnetic field strength in the ALEXIS device, a continuous variation of the ratio of the ion gyro-radius to the shear scale length is observed. As a result, a transition of the shear flow driven instability regime is also observed, which is reminiscent of the satellite observations of broadband electrostatic noise. For the first time, a laboratory experiment has reproduced the actual space observation of broadband emission, which is a characteristic signature of boundary layer crossings by a satellite. iii Acknowledgments I would like to start by thanking my advisor Dr. Edward Thomas, for all of his support throughout my time here at Auburn. Without his advice and guidance, none of this would have been possible. I am so thankful for the opportunities he has granted me, from working on ALEXIS, collaborating with so many wonderful people, and traveling to so many wonderful new places across the country and around the world to attend conferences. I am also greatly appreciative of the advice and direction provided by Dr. Bill Amatucci and Dr. Guru Ganguli at the Naval Research Laboratory. Parts of this research would have been nearly impossible without their involvement. Additionally, I am very grateful for all of the support that the physics department faculty members have provided over the years. It was a privilege to be able to learn from and work with everyone in this department. My time in Auburn would not have been as productive or enjoyable without the company of my fellow graduate students Ashley Eadon, Ross Fisher, Jeff Herfindal, Aaron Modic, Mihir Pandaya and Ivan Arnold. Thank you for all of the invaluable advice and encouragement you have given over the years. From studying for qualifying exams to struggling with experiments, each one of you made the journey through graduate school easier, and at many times entertaining. I definitely would not have made it out in one piece without the company of you guys. iv Finally, I would like to dedicate this dissertation to my family who has always encouraged my curiosity of science. I would like to thank my parents for all of the support and advice throughout the years, and for believing in me even when I didn’t. Without you guys, my larger than life dreams would have never come true. Thank you to my grandparents, who helped me in any and every way they could throughout the last 10 years of school and always had a room open for me when I needed a few days just to get away and go to the beach. It would have been impossible to make it this far without all of your support and encouragement! v Table of Contents Abstract ......................................................................................................................................... ii Acknowledgments........................................................................................................................ iv List of Figures ............................................................................................................................... x List of Tables ............................................................................................................................. xix Chapter 1: Introduction ................................................................................................................. 1 1.1: Sheared plasma flows ...................................................................................................... 1 1.2: Experiment motivation .................................................................................................... 6 1.3: Dissertation outline ........................................................................................................ 14 Chapter 2: Theory ....................................................................................................................... 16 2.1: Lower hybrid instabilities .............................................................................................. 16 2.2: Electron-ion hybrid instability ....................................................................................... 20 2.2.1: Perturbed ion density ............................................................................................ 24 2.2.2: Perturbed electron density .................................................................................... 28 2.2.3: Dispersion relation ................................................................................................ 30 2.2.4: Shooting code ....................................................................................................... 34 2.3: Whistler wave ................................................................................................................ 39 2.4: Summary ........................................................................................................................ 40 Chapter 3: Experimental Design and Hardware ......................................................................... 42 3.1: Vacuum vessel ............................................................................................................... 42 vi 3.2: Electromagnet configuration .......................................................................................... 45 3.3: Vacuum pumps .............................................................................................................. 46 3.4: Gas regulation ................................................................................................................ 48 3.5: Plasma generation and modification .............................................................................. 49 3.6: Diagnostics..................................................................................................................... 54 3.6.1: Emissive probe ...................................................................................................... 55 3.6.2: Langmuir probes: single and double probe.......................................................... 57 3.6.3: “k” – probe ............................................................................................................ 63 3.6.4: Magnetic Loop (B-dot) probe ............................................................................... 69 Chapter 4: Experimental Results ................................................................................................ 76 4.1: Dual plasma parameter measurements .......................................................................... 77 4.2: Electron-ion hybrid instability experiments .................................................................. 85 4.2.1: Density gradient modified electron-ion hybrid instability .................................... 85 4.2.1.1: Modification of the electric field profile ...................................................... 91 4.2.1.2: Electrostatic instability characterization .................................................... 101 4.2.2: Electron-ion hybrid instability in a uniform density ........................................... 112 4.3: Ion gyro-radius transition experiments ........................................................................ 122 4.3.1: Introduction

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